MODE OF REPRODUCTION IN RELATION TO BREEDING 49 



cross between different varieties a large percentage of the plants 

 are homozygous and will breed true for their characters if self- 

 fertilization is continued. The number of different biotypes 

 which can be isolated from a cross depends upon the number of 

 allelomorphic pairs of factors involved and their linkage relations. 

 Formerly the heterozygous condition was believed to carry 

 with it an increased developmental stimulus. It was also believed 

 that this stimulus was greater when the mate to an allelomorphic 



100* 



Percentage of Heterozygous 

 Individuals in each Selfed 

 Generation when the Number 

 of Allelomorphs Concerned 

 Are: 1,5,10,15. 



456 



Segregating Generations 



10 



FIG. 12. The percentage of heterozygous individuals and the percentage of 

 heterozygous allelomorphic pairs in the whole population in each generation 

 of self-fertilization. (After Jones.) 



pair was lacking than when both were present. The physiolog- 

 ical cause of this growth stimulus was not known although it 

 was recognized that "the greater the degree of heterozygosis the 

 greater is the vigor of the resulting plant" (East and Hayes, 1912). 

 A considerable number of studies showed that the rapidity and 

 amount of cell division was increased. 



A Mendelian explanation of this growth stimulus which is so 

 frequently found in crosses, has been advanced. Jones (1918) has 

 explained the vigor of F\ which has been called heterosis on the 



4 



